The development of mid-infrared (IR) waveguides has been driven by their use as remote or small-sample-size chemical sensors for surface sensitive spectroscopy. Such waveguides can be thought of as miniaturized multiple reflection elements (MREs) wherein the incident light undergoes total internal reflection at the interface between media of different refractive indices. At each internal reflection within the waveguide, a portion of the optical field, the evanescent wave, extends beyond the high-index waveguide into the adjacent low-index medium, to a depth (d.sub.p) dependent on the angle of incidence and the ratio of the two refractive indices..sup.1 The ability of molecules outside the high-index waveguide, but near its surface, to absorb energy travelling through the waveguide via this evanescent wave makes possible the phenomenon known as attenuated total reflection (ATR) or evanescent-wave spectroscopy (EWS).
In the IR region, high-refractive-index materials as Ge, Si, and KRS-5 (Tl.sub.2 BrI), cut and polished as prisms having trapezoidal or parallelogram cross-sections and dimensions on the order of 50.times.10.times.2 mm, are in common use for EWS measurements. These macroscopic waveguides typically have throughputs matched to commercial FTIR spectrometers, i.e. in the vicinity of 1-10 mm.sup.2 -stearadian. Commercially available IR fiber optics (multimode cylindrical waveguides made of, e.g., chalcogenide glass),.sup.2,3 have more recently been used as EWS sensors..sup.4,5 These optical fibers typically have much lower throughputs than the prism MREs, complicating somewhat their use with commercial IR spectrometers. Nevertheless, when properly coupled to a small-area (low-noise) IR detector, fiber optics display the advantage that miniaturization allows smaller amounts (.mu.L) of sample to be detected..sup.6-9 This advantage arises from the fact that, while the surface sensing area is smaller, the light experiences a larger number of reflection per unit length of waveguide, yielding a concomitant increase in evanescent path length. It would be desirable to see how far this advantage could be extended, i.e. how thin an EWS waveguide or fiber could be made. However, it becomes impractical to make a free-standing IR fiber less than .about.50 .mu.m in diameter. Thus, we are interested in investigating the possibility of using supported thin planar waveguides for EWS applications.
Most thin planar waveguide development has been in the visible region, where low-loss transparent materials (polymers and glasses) are commercially available and easy to manipulate. Such waveguides have generally been used in conjunction with single-frequency lasers, which provide high luminosity, monochromaticity, and fine control over the launch angle, and have been used for absorption, Raman, and fluorescence analytical methods..sup.10-17 In contrast, IR-transmissive materials with the requisite high refractive indices and low attenuation values are either very brittle, or have not had techniques developed to allow them to be deposited (e.g. by evaporation or sputtering techniques) as uniform and well-adhered films of the desired thicknesses of 1-100 .mu.m..sup.18 To our knowledge, there have been only two reports in the literature of mid-infrared transmitting planar waveguides, both utilizing monochromatic (laser) light to demonstrate coupling and guiding, rather than broadband light which is likely to be more suitable for general chemical sensing..sup.19,20
Nevertheless, the same theory should describe both visible and infrared waveguides, and this theory indicates that with appropriate coupling methods it should be possible to use IR supported planar waveguides to perform broadband IR chemical sensing..sup.21 We have now proven this by fabricating supported planar waveguides as thin as 30 .mu.m with good broadband transmission, which demonstrate many of the characteristics predicted by planar waveguide theory..sup.22 In particular, they show a great increase in the sampling sensitivity as compared to previous evanescent-wave absorption measurements.